1. Field of the Invention
This invention generally concerns robotic systems and is specifically concerned with an improved robotic light-weight system for servicing the heat exchanger tubes of a nuclear steam generator.
2. Related Art
In a pressurized water nuclear powered electric generating system, the heat generated by the nuclear reaction is absorbed by a primary coolant that circulates through the reactor core and is utilized to generate steam in a steam generator. The steam generator typically is an upright cylindrical pressure vessel with hemispherical end sections. A traverse plate called a tube sheet, located at the lower end of the cylindrical section, divides the steam generator into a primary side, which is the lower hemispherical section below the tube sheet, and a secondary side above the tube sheet. A vertical wall bisects the primary side into an inlet section and an outlet section. The tube sheet is a thick carbon steel plate with an array of thousands of holes into which are inserted the ends of U-shaped tubes. One end of each U-shaped tube is inserted into a hole within the tube sheet which communicates with the inlet section of the primary side and the other end is inserted in a hole within the tube sheet which communicates with the outlet section. The primary coolant is introduced under pressure into the inlet section of the primary side, circulates through the U-shaped tubes and exits through the outlet section. Water introduced into the secondary side of the steam generator circulates around the U-shaped tubes and is transformed into steam by heat given up by the primary coolant.
Occasionally during the operation of the steam generator, degradation occurs in some of the tubes. This is undesirable because the primary coolant is radioactive and any leakage of the reactor coolant into the secondary side of the generator contaminates the steam. It is generally not practical, however, to replace degraded tubing, but instead the steam generator is periodically inspected and the effected tubes are plugged at both ends. In view of the thousands of tubes in the steam generator, plugging of a few tubes does not appreciably affect the efficiency of the heat transfer.
Because of the radiation hazard present in steam generators used in a nuclear powered utility, the heat exchanger tubes of such steam generators must be, for the most part, remotely serviced to avoid exposing maintenance personnel to potentially harmful radiation. Consequently, a number of robotic systems have been developed for remotely performing repair and maintenance operations on these heat exchanger tubes. These robotic systems typically include some sort of robotic delivery arm in combination with any one of a number of specialized tools designed to be carried by the robotic arm, which are known in the art as “end effectors”. These robotic systems mostly fall into two basic categories, which shall be referred to in this application as full movement arms, and limited movement arms. Full movement arms are capable of maintaining an end effector at a desired orientation while moving it along a trajectory having components in all three spatial axes. By contrast, limited movement arms generally are capable of moving an end effector only along a selected two-dimensional trajectory, and cannot maintain the end effector at a desired orientation along this trajectory. The mechanical action of limited movement robotic arms often resembles the operation of a compass used to draw circles, i.e., one end of the arm is pivotally mounted at a point on a flat tube sheet within the channel head of the nuclear steam generator, while the middle portion of the arm is telescopically extendable or retractable. Such arms are capable of sweeping their distal, tool-holding ends across any one of a number of arcs of greater or lesser radii which intersect with desired delivery points on the tube sheet. An example of such a limited movement robotic arm is the model SM-22 arm manufactured by Zetech located in Isaquah, Wash.
Full movement arms differ from the relatively simple structure of the previously described arms in that they include six different segments which are articulated at six different motor driven joints, which in turn allows movement around six different axes. The more complex structure of such robotic arms allow them to use three of their axes of movement to hold an end effector at a desired orientation, and the other three axes to move the end effector across an infinite number of trajectories in three dimensions while maintaining the end effector at the desired orientation. Such abilities are highly advantageous in situations where it is essential to maintain the end effector at a constant orientation during a servicing operation, as is often the case with a weld head being moved around the location of a desired weld seam. While limited movement arms are often necessarily dedicated to the delivery and manipulation of a single end effector, such as for example a tube inspection probe, full movement arms have the ability in theory to couple onto and decouple from a variety of end effectors. One of the most advanced designs of such a full movement robotic arm is the ROSA (Remotely Operated Service Arm) developed by Westinghouse Electric Corporation located in Pittsburgh, Pa.
Unfortunately, neither of these types of robotic arm is without drawbacks. While limited movement robotic arms are relatively simple and inexpensive to construct and to install in nuclear steam generators, the fact that they are typically dedicated to a single end effector necessitates the installation and removal of a number of such arms to complete inspection and servicing operations on the heat exchanger tubes within the channel head of the steam generator. This is a significant shortcoming as every such installation procedure is not only laborious and time-consuming, but also results in the exposure of the operating personnel to potentially harmful radiation. This last drawback is of growing importance, as the NRC has placed greater limitations upon the amount of radiation exposure that such operating personnel may absorb. Moreover, the fact that such limited movement robotic arms can not maintain an end effector at a desired orientation while simultaneously moving it across a chosen trajectory across the tube sheet renders them useless for end effectors that require a constant orientation, such as weld heads. Of course, full movement robotic arms such as the aforementioned ROSA are not limited in these ways. However, prior art full movement robotic arms such as ROSA also have limitations that offer room for improvement. Specifically, the Applicants have noticed that a vertically oriented “elbow” of the ROSA disadvantageously limits the number and length of the possible trajectories that the distal end of the arm may make without mechanically interfering with the bowl-like wall of the channel head, or the divider plate within the channel head, or the cables which vertically drape down from end effectors such as the eddy current probes used to inspect and determine the condition of the interior walls of the heat exchanger tubes. Applicants have also observed that the prior art ROSA is configured so that a large portion of the arm is cantileverly supported from its vertically oriented elbow, which in turn applies a significant amount of life-reducing extraneous torque to the electric motor driving the joint, and reduces its payload carrying capability. Further, Applicants have observed that the motion of the distal end of such robotic arms is not smooth enough to conduct certain weld operations.
Servicing of nuclear steam generators has changed dramatically in the last twenty (20) years. In the past, much of the older steam generator tubing became degraded and required significant plugging, sleeving, or total steam generator replacement. Power plant service outages were of long duration and the repairs to steam generators required sophisticated robotic manipulators such as ROSA and tooling with the capability to perform varied inspections and repairs while avoiding plugged tubes.
Presently, most utilities in the industry have either replaced their steam generators or have generators with few plugged tubes and require minimal repairs. Most of the work for the steam generator tubing involves eddy current inspection with few or no plugs required. Outage time for plant maintenance and refueling, which dictates the length of the outage, is reduced, significantly decreasing the time allotted for steam generator inspection and service. There is a clear need to improve upon existing robotic systems for servicing steam generators to meet the current needs. Current steam generator manipulators are generally heavy (greater than 100 lbs.) and sophisticated, which adds time and personnel radiation exposure to transport, set up, and install in the steam generators. Current manipulators are also too large in size to install multiple manipulators in either section of most steam generator channel heads. Use of a single manipulator to position more than two eddy current probes simultaneously has not been very successful because failure of one probe will generally result in the same number of robotic moves as if one or two probes were used. Additionally, steam generator robotics generally have three or more degrees of freedom along with multiple motors with position feedback. The control system along with the complexity of the manipulator are generally expensive to purchase and maintain. Furthermore, most manipulators, when installed in the steam generator, are anchored and must be moved to gain access to all the tubes during inspection. In addition, position adjustment and verification is usually required due to the varied deflection of the manipulator under load coupled with inaccuracy of the robot.
Accordingly, a simple, small, light-weight robot is desired that can function as a steam generator inspection and/or plugging manipulator. Preferably, such a manipulator has a weight of approximately 30 lbs. or less and should have an approximately 70 lb. or greater payload capacity.